1. Field of the Invention
The present invention relates to a method for making piezoelectric ceramic elements used for piezoelectric resonators, piezoelectric actuators, piezoelectric filters, piezoelectric buzzers, piezoelectric transformers, etc., and more particularly, to a method for making a monolithic piezoelectric ceramic element in which a conductor containing Ag as a principal ingredient is used as an internal electrode.
2. Description of the Related Art
With respect to piezoelectric ceramic elements used for piezoelectric resonators, piezoelectric actuators, piezoelectric filters, piezoelectric buzzers, piezoelectric transformers, etc., it is known that firing under the optimum conditions according to the desired use is absolutely necessary to provide elements having excellent characteristics with high reliability, without structural defects. Various attempts have been made for that purpose.
For example, Japanese Unexamined Patent Application Publication No. 2-74566 discloses a process for firing the material while maintaining the oxygen concentration of the furnace atmosphere at 50% by volume or more during the heating period to raise the temperature to the firing temperature, and maintaining the oxygen concentration of the furnace atmosphere between half of the oxygen concentration during the heating process and 10% by volume during the firing temperature retention period. During the heating period in the atmosphere with a high oxygen concentration of 50% by volume or more, the oxygen concentration in the closed pores formed during this period is increased, and during the firing temperature retention period, when the oxygen concentration is controlled between half of the oxygen concentration during the heating period and 10% by volume, the difference in oxygen concentration between the inside of the closed pores and ambient air is increased. Therefore, the oxygen diffusion velocity is increased, and piezoelectric ceramics comprising oxides can be sintered densely in large quantities.
Japanese Unexamined Patent Application Publication Nos. 4-357164 and 10-95665 disclose processes in which firing is performed in an atmosphere with an oxygen concentration of 80% by volume or more throughout the firing profile. Consequently, it is possible to obtain piezoelectric elements, and piezoelectric resonators in particular, in which pores in ceramic grains and structural defects are inhibited, thus being dense and highly reliable.
A process is also known in which firing is performed in air, i.e., in an atmosphere with an oxygen concentration of approximately 21% by volume throughout the firing profile regardless of the types of internal electrodes. This process is employed for various piezoelectric elements.
When a green laminate formed by stacking ceramic green sheets to which an internal electrode conductive paste containing Ag as a principal ingredient is applied is fired in the conventional firing process for piezoelectric ceramic elements, such as the ones as described above, however, the Ag contained in the internal electrodes is incorporated into the ceramic grains in the piezoelectric ceramic layers, resulting in degradation in piezoelectricity.
Lower profile piezoelectric elements have been developed in order to meet higher frequencies in the technical field of piezoelectric elements. When the profile is lowered, although the thickness of the piezoelectric element is decreased, satisfactory mechanical strength is also required in order to prevent the piezoelectric element from being damaged by high-frequency mechanical vibration.
It is an object of the present invention to provide a method for making a monolithic piezoelectric ceramic element having high mechanical strength, excellent piezoelectricity and high reliability.
In accordance with the present invention, a method for making a monolithic piezoelectric ceramic element includes the steps of applying a conductive paste including a conductor containing Ag as a principal ingredient to ceramic green sheets comprising a piezoelectric ceramic material containing a Pb compound, stacking the ceramic green sheets to form a laminate, and firing the laminate in an atmosphere with an oxygen concentration of about 90% by volume or more during the heating period and with an oxygen concentration of about 5% to 15% by volume during the isothermal period (firing temperature retention period) and during the cooling period.
The oxygen concentration is set at about 90% by volume or more during the heating period in the firing process because if the oxygen concentration is about 90% by volume or more, it is possible to increase the oxygen concentration in pores formed in the ceramic constituting the piezoelectric ceramic element, and oxygen is thereby diffused to suppress the pores in the ceramic grains and the structural defects. As a result, a densely sintered compact can be obtained.
However, if the oxygen concentration is lower than about 90% by volume in the pores formed in the ceramic constituting the piezoelectric ceramic element, it is not possible to decrease the pores satisfactorily since the oxygen concentration is insufficient. As a result, a densely sintered compact cannot be obtained, and the transverse strength of the ceramic is decreased.
The oxygen concentration is set in the range of about 5% to 15% by volume during the isothermal period and during the cooling period because if the oxygen concentration is about 5% to 15% by volume, it is possible to decrease the Pb content in the grain boundaries of the ceramic constituting the piezoelectric ceramic element, and characteristics of the piezoelectric ceramic element, i.e., piezoelectricity and reliability, can be improved.
In an atmosphere with a high oxygen concentration of 90% by volume or more during the heating period, the Pb vapor pressure is increased and evaporation of Pb from the piezoelectric ceramic element itself is decreased, and redeposition of Pb from the atmosphere also occurs, resulting in an increase in the Pb content in the ceramic grain boundaries. In the subsequent atmosphere with a low oxygen concentration of about 5% to 15% by volume during the isothermal period and during the cooling period, since evaporation of Pb which is present in the grain boundaries of the ceramic constituting the piezoelectric element is accelerated, the Pb vapor pressure is decreased and the impurity content in the grain boundaries is decreased. Therefore, the characteristics of the piezoelectric element itself are improved.
If the oxygen concentration exceeds about 15% by volume, the effect of decreasing the Pb content in the ceramic grain boundaries is lost, resulting in degradation in piezoelectricity.
If the oxygen concentration is lower than about 5% by volume, the piezoelectric element itself is reduced, resulting in degradation in piezoelectricity.
In the case of lead zirconate titanate (PZT)-based compound, firing is performed during the heating period in an atmosphere with an oxygen concentration of approximately 90% by volume or more. The firing temperature in the isothermal period, i.e., in the maximum temperature range, is set at the temperature when sintering of the laminate is complete in the atmosphere, i.e., specifically, is set at the temperature when the pack density of the monolithic sintered compact is 99% or more of the theoretical density. Additionally, the retention time in the maximum temperature range is also set under the same condition as that for the firing temperature.
Each of the heating rate and the cooling rate is set in the range of about 1xc2x0 C. to 10xc2x0 C. per minute, and the retention time in the maximum temperature range is set in the range of about 1 to 10 hours.
However, the firing conditions are not limited to those described above. Optimum firing temperature, heating rate, retention time and cooling rate may be selected depending on the type (composition) of the piezoelectric ceramic material containing the Pb compound.
As the piezoelectric ceramic material for the ceramic green sheets, in addition to the lead zirconate titanate (PZT)-based compound, a lead titanate (PbTiO3)-based compound, a lead metaniobate (PbNb2O6)-based compound, a lead metatantalate (PbTa2O6)-based compound, or the like may be used.
The conductor included in the conductive paste for internal electrodes contains Ag as a principal ingredient and can contain Pd content of about 50% by weight or less.